I think taxing Canadian citizens if they spend too much time in the US is utterly ridiculous. They are not US citizens, and as such, should not be subject to US taxes. Their income has not been earned in the US, in the sense that they performed work in the US.
This reminds me of my time in the military, when I was stationed in California. I learned that California was trying to tax military retirement income if the military member served any time in California. In my case, I served 4 years, so, theoretically 1/5 of my retirement income was potentially going to be taxed by California. My wife worked in California as well. Once I knew this, I REFUSED to file a state income tax return. I would calculate it, and fill it out, to make sure I didn't owe, but I would not file. I was due refunds, but didn't care. They were NOT going to get my SSAN associated with any state tax return.
Hate this **** from my government.
Mike